Mass Spectrometry is a wide diffuse technology for the analysis of various polar and not polar compounds. In particular, Liquid Chromatography has been employed in the analysis of compounds with different polarity degree and molecular weight. The characterization and quantitation of these compounds are, in fact, of interest and new methodologies are continuously developed for their analysis. In the recent years various technologies have been developed for analyzing various molecules by Mass Spectrometry. For example, the analysis of addict drugs is one of the recent fields where Liquid chromatography-mass spectrometry has given strong improvement (Cristoni S, Bernardi L R, Gerthoux P, Gonella E, Mocarelli P. Rapid Commun. Mass Spectrom. 2004; 18: 1847; Marquet P, Lachatre G. J. Chromatogr. B Biomed. Sci. Appl. 1999; 73: 93; Sato M, Hida M, Nagase H. Forensic Sci. Int. 2002; 128: 146). In particular this technique has permitted to directly analyze addict drug compounds in urine samples without subjecting them to the derivatization reaction (Cristoni S, Bernardi L R, Gerthoux P, Gonella E, Mocarelli P. Rapid Commun. Mass Spectrom. 2004; 18: 1847). This reaction is, in fact, necessary to analyze these compounds when the gas-chromatography mass spectrometry technique (GC-MS) is employed, increasing the costs of the analysis. Another field of interest is the analysis of macromolecules like proteins, peptides and oligonucleotides (Kim S Y, Chudapongse N, Lee S M, Levin M C, Oh J T, Park H J, Ho I K. Brain Res. Mol. Brain Res. 2005; 133: 58; Cristoni S, Bernardi L R. Mass Spectrom. Rev. 2003; 22: 369; Cristoni S, Bernardi L R, Biunno I, Tubaro M, Guidugli F. Rapid Commun. Mass Spectrom. 2003; 17: 1973; Willems A V, Deforce D L, Lambert W E, Van Peteghem C H, Van Bocxlaer J F. J. Chromatogr. A. 2004; 1052: 93.). Once these molecules have passed through an ionization source, the charged molecules are analyzed using a mass spectrometric analyzer (Ion Trap (IT), Time Of Flight (TOF), Fourier Transform Ion Cyclotron Resonance (FTICR), Quadrupole, Triple Quadrupole (Q1Q2Q3) etc).
The ionization source is a key component of the mass spectrometer. It transforms neutral molecules into ions which can be analyzed by mass spectrometry. It must be stressed that various ionization sources are employed to ionize the analytes because of the fact that various physicohemical ionizing effect must be used depending on the physicochemical behavior of the compound to be ionized. Actually, the most used ionization sources are Electrospray (ESI), Atmosheric Pressure Chemical Ionization (APCI) and Matrix Assisted Laser Desorption Ionization (MALDI) techniques that are highly effective for the production of ions in the gas phase, to be subsequently analyzed by Mass Spectrometry (MS) (Cristoni S, Bernardi L R. Mass Spectrom. Rev. 2003; 22: 369). While ESI and APCI operate on liquid samples, MALDI is used to analyze solid state samples.
In the case of ESI a strong electric field is used to both vaporize and ionize the analyte. In this case multi-charge ions (one molecule gives rise to more than one signal) of medium/high molecular weight compounds (like proteins and oligonucleotides) are produced. The mass spectra so obtained are difficult to analyze and specific software algorithms can be used for data analysis (Pearcy J O, Lee T D. J. Am. Soc. Mass Spectrom. 2001; 12: 599; Wehofsky M, Hoffmann R. J Mass Spectrom. 2002; 37: 223). Low molecular weight compounds give usually rise to a mass spectrum simple to analyze due to the formation of mono-charged ions (one molecule gives rise only to one signal). Thus, this ionization source is mainly used to analyze medium- and high-polar compounds having low-, medium- or high-molecular weight.
In the case of APCI the sample is first gasified at high temperature (250-500° C.) and then ionized through the corona discharge effect produced by a needle placed at high potential (2000-8000 V). This ionization approach can be used to analyze low molecular weight compounds (molecular weight<600 Da) having medium low polarity (e.g. steroids etc).
In the case of MALDI low charge state molecules are produced (typically mono- and bi-charged ions). In this case the analyte is co-crystallized with a matrix compound able to adsorb ultraviolet (UV) light with a wavelength of 337 nm. The co-crystallized sample is then placed in a vacuum region (10−8 torr) and irradiated with a 337 nm UV laser light. A micro-explosion phenomenon, named “ablation” takes place at the crystal surface so that analyte and matrix are gasified. Moreover, the analyte is ionized by various reactions that typically takes place between analyte and matrix. This approach is usually employed to analyze high molecular weight compounds having various polarities.
All the above described ionization approaches are not suitable to analyze non-polar compounds like benzene, toluene etc. For this reason a new ionization source named Atmospheric Pressure Photo Ionization has been developed and employed to analyze various compounds (Raffaelli A, Saba A. Mass Spectrom Rev. 2003; 22; 318). As in the case of APCI the liquid sample solution is gasified at high temperature. The analyte is then irradiated by a UV light (10 ev Kr light) and ionized through various physicochemical reactions (mainly charge and proton exchange and photoionization reactions).
A new ionization approach, named “Surface Activated Chemical Ionization—SACI” has been also recently developed in order to improve the performance of the commercially available mass spectrometer in the analysis of various kind of compounds extracted from biological matrix (PCT No WO 2004/034011). This apparatus is based on the introduction of a surface for the ionization of neutral molecules in an atmospheric pressure chamber. SACI has been obtained by upgrading the Atmospheric Pressure Chemical Ionization (APCI) source (Cristoni S, Bernardi L R, Biunno I, Tubaro M, Guidugli F. Rapid Commun. Mass Spectrom. 2003; 17: 1973). In fact, it was observed that introducing into the APCI ionization chamber an element carrying a plate-like active-surface can bring to unexpected results in terms of high sensitivity and possibility to detect molecules having a molecular weight in a broad range of values (Cristoni S, Bernardi L R, Biunno I, Tubaro M, Guidugli F. Rapid Commun. Mass Spectrom. 2003; 17: 1973; Cristoni S, Bernardi L R, Gerthoux P, Gonella E, Mocarelli P. Rapid Commun. Mass Spectrom. 2004; 18: 1847; Cristoni S, Sciannamblo M, Bernardi L R, Biunno I, Gerthoux P, Russo G, Chiumello G, Mora S. Rapid Commun. Mass Spectrom. 2004; 18: 1392).
However, there is no ionization source able to softly ionize all compounds. This is mainly due to their different physicochemical proprieties, thus, different physicochemical effects must be employed in order to give rise to the analyte ionization.